By representing entities, their cardinality relationships (e.g., one-to-one, one-to-many, etc.), and the keys that connect them, an Entity-Relationship (E-R) diagram is both a design tool and a map that shows how to use the data. The construction of an E-R diagram is essential for the design of tables, of extracts and of metadata. E-R diagrams are especially important in the designing and developing data documentation and of tables that represent history. E-R diagrams can greatly facilitate the maintenance of the warehouse and of the data extracts that populate it.